Figure 9. Areas where activations were stronger during vowel discrimination than vowel memory tasks (blue) and areas where activations were stronger during vowel memory than vowel discrimination tasks (red).
3.5 STUDY III. ACOUSTICAL AND CATEGORICAL TASKS DIFFERENTLY MODULATE ACTIVATIONS OF HUMAN AUDITORY CORTEX TO VOWELS
3.5.1 VOWELS
Three groups of phonemic (/i–y/, /u–o/ and /æ–a/; 30, 28 and 45 vowels in each group, respectively) and nonphonemic vowels (NPh1, NPh2 and NPh3;
12 vowels in each) were synthesized (Figure 10a). In each group, the vowels were separated by 60 mel in either F1, F2 or both. In order to define individual phoneme categories and category boundaries, subjects classified the vowels in each phonemic vowel group into two phonemes. Based on the results, three phoneme boundaries (/i/–/y/, /u/–/o/ and /æ/–/a/) and six corresponding phoneme categories were defined. Next, subjects rated the goodness of the vowels in these categories. The vowel with the highest rating in each category was selected as the prototype and the vowel with the lowest rating as the nonprototype. These vowels were then paired with adjacent vowels (within the same category) to construct prototype or nonprototype vowel pairs (Figure 10b). Further, cross-category vowel pairs were constructed based on the individually defined phoneme boundaries. In the cross-category vowel pairs, one vowel was a nonprototype next to the vowel
boundary and the other vowel was an adjacent nonprototype vowel at the same or opposite side of the phoneme boundary (b).
3.5.2 TASKS AND STIMULUS STREAMS
Study III compared activations to prototypical, nonprototypical and nonphonemic vowels during vowel discrimination, vowel 2-back, category discrimination, and visual tasks (Figure 10c–f).
Prototype, nonprototype, nonphonemic and cross-category vowel pairs were presented in separate task blocks. Vowel discrimination, vowel 2-back memory and visual tasks with Gabor gratings were similar to the tasks used in Study II. In the category discrimination task subjects were required to indicate when both parts of the vowel pair belonged to the same phonemic category.
Each task block contained vowels from all the three vowel categories (/i/, /u/ or /æ/), vowel groups (/i–y/, /u–o/ or /æ–a/) or nonphonemic categories (NPh1–3). In all cases, the vowels in a pair were either the same or separated by approximately 60 mel (in F1, F2 or both).
Figure 10. Experimental design. (a) In 13 s blocks (alternating with 8 s rest with no stimuli), subjects were presented with vowel pairs from three Finnish phonemic vowel continua (blue, red and green) and three nonphonemic (NPh) vowel categories (gray) defined in F1/F2 space. (b) Individually defined category boundary between /i/ and /y/ vowels (gray dashed line), prototype (P, diamond) /i/, nonprototype (NPr, square) /i/, and three nonprototype /i/ and two /y/ vowels (circle) near the category boundary. Examples of prototype (Pr + adjacent vowel), nonprototype (nonprototype + adjacent vowel) and cross-category vowel pairs are shown (a thin line connects vowels in a pair). (c) In the vowel discrimination and (d) in the vowel 2-back memory task, subjects were presented with prototype, nonprototype or nonphonemic vowel pairs. In the vowel discrimination task subjects were required to indicate when the vowels in a pair were the same.
In the vowel 2-back memory task, subjects indicated when the vowel pair belonged to the same vowel category as the one presented 2 trials before. (e and f) Three different tasks were performed on cross-category and within-category vowel pairs (CC) which comprised of vowels
near the category border. In the discrimination task, the target was a pair in which the parts were identical (e). In the category discrimination task, the target was a pair in which both parts belonged to the same vowel category (but were not always identical vowels) (e). In the 2-back tasks, the target was a vowel pair that belonged to the same vowel group (/i–y/, /u–o/ or /æ–a/) as the pair presented two trials before (f). In e and f, the markers represent different vowels at a similar distance from the vowel category border.
3.5.3 RESULTS AND DISCUSSION
Consistent with Study I, stronger activations in AC were observed for prototype than for nonprototype vowel pairs (Figure 11a) during discrimination task. However, while in Study I the activation enhancements to prototype pairs were restricted to areas near HG, in Study III these activation enhancements were observed in wider areas in STG and IPL. In Study III, the difference between prototype and nonprototype blocks was also observed during the vowel 2-back task (b). As in Study I, only minor stimulus-dependent activation differences between prototype and nonprototype vowel blocks were observed when the prototype and nonprototype vowels were presented during visual tasks (i.e., no directed auditory attention; c). Thus, stimulus-dependent activations alone cannot explain the activation differences observed between prototype and nonprototype vowel blocks during active listening tasks.
Figure 11. Activation differences during discrimination, 2-back and visual tasks performed on nonprototype (NPr) and prototype (Pr) vowel pairs.
Consistent with Study II, stronger activations in AC were observed for nonphonemic than for phonemic (nonprototype) vowel pairs during discrimination task (Figure 12a). This effect was also present during 2-back task (b). Furthermore, nonphonemic vowel blocks were associated with stronger stimulus-dependent activations (during visual task) than phonemic blocks (c). These stimulus-dependent effects were probably due to the fact that in Study III there were more different vowels in nonphonemic (vowels in nonphonemic pairs were randomly selected) than in phonemic (the pairs of phonemic vowels were organized around a nonprototype) blocks. These stimulus-dependent effects in medial STG, however, cannot explain the differences between phonemic and nonphonemic vowel blocks in more lateral STG areas during auditory tasks. Thus, Study III successfully replicated the main findings of Studies I and II showing that AC activations during active tasks are sensitive to the language-level differences between prototype and nonprototype as well as between phonemic and nonphonemic vowels.
Figure 12. Activation differences during discrimination, 2-back and visual tasks performed on nonprototype (Npr) and nonphonemic (Nph) vowel pairs.
Consistent with Study II, vowel discrimination and vowel 2-back tasks enhanced activations in anterior-posterior STG and IPL, respectively (Figure 13a). These task-dependent activation patterns were quite similar irrespective of whether the tasks were performed on prototype, nonprototype, nonphonemic or cross-category vowel pairs. Study III also compared activations to identical cross-category (CC) vowel pairs presented during discrimination, 2-back memory and category discrimination tasks (b, c). In general, vowel discrimination and category discrimination were
associated with quite similar activation patterns in STG regions. The category discrimination task, however, was associated with enhanced activations in IPL (b). Further, although quite similar IPL areas were activated during the category discrimination and 2-back tasks, the category discrimination task was associated with stronger activations in areas of the insula and STG, whereas during the 2-back task activations were stronger in IPL (c). These activation patterns observed during three different tasks performed on identical stimuli show that activations in areas of AC and IPL strongly depend on whether the task requires analysis of the acoustical or categorical features of the sounds. More specifically, these results support the view of Rinne and colleagues (2009, 2012) that during listening tasks areas of STG are implicated in analysis of detailed acoustic information, whereas activations in IPL are associated with operations on categorical representations.
Figure 13. (a) Comparison of activations during all vowel discrimination (blue) vs. all vowel 2-back (red) tasks. (b) Activation differences during discrimination and category discrimination and (c) during category discrimination and 2-back tasks performed on identical cross-category (CC) vowel pairs.
4 GENERAL DISCUSSION
The results of the present thesis are consistent with the view that phonemic vowels are represented in a categorical manner and that categorical vowel information is available in human AC during active listening tasks. First, Studies I and II found that activation in AC is sensitive to the language-level difference between prototypical and nonprototypical phonemic vowels and between phonemic and nonphonemic vowels. Importantly, these results were successfully replicated in Study III. The results of the present thesis also shed new light on the role of IPL in categorical processing. The results implicate IPL in tasks requiring operations on categorical representations rather than categorization as such.